Recovering fresh water from brine, such as sea water, is desirable to supplement the limited fresh water supply of the world. Technologically, the separation of fresh water from brine is relatively easy, but to effect such separation on an economic basis that would justify a process being used for municipal, industrial or agricultural supplies is difficult. Not only must such a process be economically justified, it must even be justified from an environment conservation viewpoint because the use of too much energy to recover fresh water from brine would produce a net detrimental effect on the ecology.
As used in this specification and the appended claims, the term brine is intended to include various sources of water that would not be considered fresh water whether they are from the ocean, lakes or wells; and it is also intended to include such materials as brackish water and municipal or industrial waste waters having high concentrations of dissolved inorganic material contained in them. The term fresh water is also employed in its generic sense to mean water having a low enough impurity content to be useful as industrial, municipal or agricultural water, such as that obtained from lakes, streams or fresh water wells.
In the past, various techniques have been used to recover fresh water from brine. Examples of such processes are evaporation which separates vapor phase water from brine, freezing processes which separate solid phase water from brine, semipermeable membrane processes which produce salt-enriched brine, and a salt-depleted fresh water, and ion exchange processes which substitute innocuous ions for damaging ones, to mention a few.
All of the above processes suffer from economic rather than technological problems. For example, evaporation processes produce fresh water adequate for almost any use but they are too expensive because the corrosiveness of boiling brine requires exotic metals for construction of large vessels and pieces of equipment. In addition to the expensive capitalization of such processes, scaling and salting on heat exchange surfaces requires frequent shutdowns for maintenance which is expensive from the operation point of view. Thus, even if the energy consumed in an evaporation process is very low, approaching only the heat of solution of the salt, evaporation processes by their very nature are too expensive to produce large quantities of water for municipal, industrial or agricultural use.
Ion exchange processes and processes employing semipermeable membranes are difficult to employ on scales large enough to produce reasonable amounts of fresh water, and these processes generally produce only salt-depleted streams rather than fresh water thereby requiring many stages of purification to produce a product that has general utility as fresh water. Stagewise processes require more equipment and energy than the single-stage freezing and evaporation processes, and they have accordingly not been successful for producing fresh water from brine.
Freezing processes have long been considered a good method for recovering fresh water from brine. Freezing processes have the advantage of using low temperatures where corrosion is not a significant factor and where ordinary carbon steel vessels and equipment can be employed. In addition, freezing processes do not produce salting out of the brine of scaling on heat exchange surfaces and they produce solid ice crystals that are relatively pure water from a single stage. The primary problem with freezing processes is that it is difficult to separate the ice crystals from the liquid brine and to wash the separated crystals in a manner to produce the desired water product within reasonable economic limits.
Crystals from freezing processes ordinarily are in the form of agglomerates with liquid brine trapped between them, and even when separate ice crystals are formed, they are in such dense, impermeable beds that they are not susceptible to washing processes. Thus, even though a freezing process will produce ice crystals of relatively pure water, the liquid phase recovered from such processes is not an adequate fresh water product unless expensive further processing is employed.